WASHINGTON (CNN) -- FBI Director Robert Mueller said Tuesday he will seek an independent review of the scientific process and evidence that allowed the FBI to wrap up its long-running anthrax investigation, but left lingering questions.

In testimony on Capitol Hill on Tuesday, Mueller disclosed he has asked the National Academy of Sciences to conduct an outside probe of the cutting-edge science used by FBI investigators, and contracted research scientists which led them to anthrax researcher Bruce Ivins.

Ivins committed suicide July 29 as federal authorities were preparing to indict him with the deadly anthrax mailings which claimed five lives in 2001.

Scientists from inside and outside the FBI who worked on the investigation described in detail at a mid-August briefing how they meticulously traced a strain of anthrax from the envelopes and victims to a batch of anthrax in Ivins' lab. However, the lack of other physical evidence and eyewitnesses produced skeptics who continue to question the FBI conclusion and scientific methodology.

At the hearing Tuesday, Rep. Jerald Nadler, D-New York, demanded to know scientific details which may help end the lingering debate. Mueller said that is why he contacted the National Academy of Science.

On other issues, Mueller told the House Judiciary Committee the FBI has broadened its investigation of mortgage-related fraud to now include 24 corporations which may have misstated their assets. He said the number of individual real estate lenders, brokers and appraisers under investigation now tops 1,400 nationwide.

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On new rules adopted for FBI agents to pursue terrorists and spies, Mueller insisted the updated guidelines will help them identify enemies planning to do harm to the United States.

"I believe the American public and this committee want us to understand that potential threat and do what is necessary to try to identify persons who travel to Pakistan whatever their heritage, whatever their backgrounds, whatever their ethnicity," he said.

Several Democrats expressed concern about the revisions, and complained bitterly that they had not seen copies of the new rules.

"The more I know about them, I'm very concerned about the expanded authority to be given to FBI agents," Nadler said.

Mueller deflected questions on criminal investigations related to federal election law violations. He told House members the FBI would respond if called upon by the Justice Department civil rights lawyers to enforce voting rights.

Mueller returns to Capitol Hill on Wednesday for a similar session with the Senate Judiciary Committee.